Wire bonding has been used for an electrical connection for many years. Wire bonding is generally considered the most cost-effective and flexible interconnect technology, and is used to assemble the majority of semiconductor packages. As used herein, the term bond wire refers to the wire that provides electrical connections inside the packaging for an electronic device. By way of example, bond wires are used inside the plastic packages that house microprocessors. They provide electrical connections between the numerous, externally-visible connection pins which extend from a plastic package and connection points on the integrated circuit die inside the plastic package.
Wire diameters start at 15 μm (0.6 mils) or a little thinner, which is much thinner than a human hair and can be up to several hundred micrometers for high-powered applications. While some bond wires are made of aluminum or copper, partly because it is inexpensive or more conductive, most bond wires are made of gold in a corrosion environment because gold will not corrode and will provide a more reliable connection over time than will aluminum or copper.
Microprocessors and other electronic devices that use bond wires in some applications are subjected to vibration and/or mechanical shock. In certain applications, the frequency and amplitude of the vibration is so extreme, for example 20 Gs (1 G=9.8 meter/sec2) over 30 kHz, the bond wire's connection to a bond pad or other surface can encounter a high-cycle fatigue failure.
Experiments and computer simulations have shown that bond wire failure at the point of connection to a substrate or bond pad is mainly due to the bond wire's natural frequency resonant to the frequency of a forcing vibration applied to the device. Stated another way, if an electronic device is vibrated at a frequency that is substantially equal to the natural frequency of the bond wire, the bond wire's resultant vibration at its resonant frequency can be amplified in the order of magnitude and is likely to cause the wire to fatigue fail where it is attached to a bond pad. One prior art solution to preventing wire fatigue failure is to use aluminum wires to boost natural frequencies because aluminum has a lower mass density. Using aluminum however, requires the bonding wire to be embedded in a viscous gel to avoid wire corrosion. One disadvantage of using a gel is that a gel container or gel dam typically that is required to confine the gel can make the resulting device more complicated and more expensive. In a certain application, a specific vibration axis in a bond wire having a higher natural frequency exceeding the forcing frequency would be an improvement over the prior art in that it would avoid vibration fatigue failure. Avoiding the use of a viscous gel to reduce fatigue failure would also be an improvement over the prior art. Another advantage is also found in that a wire with an optimized profile is the shortest, or nearly the shortest, which can also save cost during high-volume manufacturing.